the new C6 and C7 idle power states that give the new Haswell architecture benefits for low power scenarios place a requirement of receiving a 0.05 amps load on the 12V2 rail. (That's just 50 milliamps!) Without that capability, the system can exhibit unstable behavior and a quick look at the power supply selector on Intel's own website is only listing a couple dozen that support the feature.

Also the owners of older Enermax power supplies can be relieved: The manufacturer applies the ZERO Load Design already in all high-end power supplies since the Revolution85+ series which has been launched in 2008.

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Seen similar sorts of replies from other manufacturers scattered around the webs. I suspect this will be mostly a non-issue for enthusiast-grade stuff. I suspect any sane mobo maker is going to disable those states by default, if only to avoid the RMA attempts.